1. Field of the Invention
The present invention relates to a digital modulated signal demodulator/decoder apparatus for demodulating and decoding a modulated reproduction signal of digital form, e.g. a reproduced signal from a magnetic recording medium, which tends to have high distortion and cause unwanted jitter and envelope variation.
2. Description of the Prior Art
In conventional digital signal magnetic recording/reproducing apparatuses, due to the difficulty of recording/reproducing the direct current component, there have been used baseband modulations having less direct component in the recording signals, such as NRZ codes (e.g., J.K.R. Heitmann "An Analitical Approach to the Standardization of Digital Video Tape Recorder," SMPTE J., 91,3, Mar. 1982, or J.K.R. Heitmann "Digital Video Recording, New Result in Channel Coding and Error Protection," SMPTE J., 93:140-144, Feb. 1984), 8-10 block codes (e.g., J.L.E. Baldwin "Digital Television Recording with Low Tape Consumption," SMPTE J., 88:490-492, Jul. 1979), Miller-square (M.sup.2) codes (e.g., L. Gallo "Signal System Design for a Digital Video Recording System," SMPTE J., 86:749-756, Oct. 1977), ternary partial response system, etc.
However, in the digital signal magnetic recording/reproducing apparatus utilizing such base band modulations, due to the basic system of recording the binary signals, utilization efficiency of the recording frequency band (transmission bit rate per unit band) is low. For instance, assuming the roll-off rate of the nykist transmission system to be 0.5, the utilization efficiency of the frequency band is at most 1.33 bits/sec./Hz. This incurs an increased tape consumption, providing a difficulty to long time recording. Further, in order to increase the recording rate, there can be considered a method of increasing the number of recording channels, or a method of increasing the relative velocity. However, if the recording frequency band is expanded, S/N ratio is extremely deteriorated. Accordingly, the recording rate cannot be increased so much. When the number of recording channels is increased, the track width is narrowed and S/N ratio is deteriorated. If the relative velocity is increased, the amount of tape consumption naturally comes into question.
To the deterioration of the S/N ratio, the error rate may be improved by using the error control coding. However, the bit rate of the data which can be actually recorded is lost by such coding (Ref. L.M.H.E. Dreissen et al., "An Experimental Digital Video Recording System," IEEE Conf. Jun. 1986, or C. Yamamitsu et al., "An Experimental Digital VTR Capable of 12-hour Recording," IEEE Trans. on CE,CE-33, No.3, pp 240-248, 1987).
On the other hand, there has been proposed a modulation scheme made by combining a multi-value modulation scheme, which shows a high utilization efficiency of the frequency band, with an error control coding for application to the digital data transmission. This, however, does not give consideration on the special conditions such as the effect of non-linear distortion or the effect of saturation characteristics in the case of making recording/producing on a magnetic recording medium (cf. G. Ungerboeck "Channel Coding with Multilevel/phase Signals," IEEE Trans. on IT, IT-28, No.1, pp. 55-67, 1982).
We, the inventors of the present invention, have invented a digital magnetic recording/reproducing apparatus employing an improved amplitude/phase modulation system which is highly enhanced in the frequency efficiency and the recording rate, as described in more detail in U.S. patent application Ser. No. 07/302,325 now Pat. No. 5,095,392. However, the amplitude non-linear characteristics and the effects of jitter and envelope variation which may be caused during playback operation of magnetic recording data have not been considered.